1. Field
The invention relates to harvesting of crops that are planted in rows, such as corn and soybeans. A vehicle is steered automatically by sensing the locations of the rows of plants. Typically, corn is planted with a predetermined spacing of from 6 inches to 14 inches, in rows about 21/2 feet apart. When the plants are harvested, the vehicle travels in a direction longitudinal of the rows of corn, automatically senses the lateral locations of the rows, and utilizes the information for fully-automatic steering or manually-assisted automatic steering.
Reference is made to a co-pending application by the inventors of this application, and presently assigned to the same assignee. It is: "Automatic Steering Apparatus Using Reflected Signals", Ser. No. 313,644, filed 2/21/89, inventors H. P. Schutten, D. B. Stephenson, and O. W. Johnson.
2. Background
Crop vehicles of this type commonly harvest twelve rows of plants simultaneously. As the vehicle moves forward the plants in the rows are guided by twelve V-shaped guides at the front of the vehicle. The V-shaped guides shepherd the flexible plants into stripping or cutting bars mounted on the crop vehicle, where the ears of corn are stripped from their stalks or the plants are severed. The V-shaped guides are typically about five feet long, and their forward tips are about 21/2 feet apart. The front of at least one of the V-guides is equipped with a pair of plant-contact sensors. These feeler-type sensors are contacted by the corn plants as they enter the V-guide. The sensors provide electrical signals that assist in the steering of the crop vehicle.
An example of only one of many prior art systems is a system that can automatically assist the steering well enough to harvest corn satisfactorily from straight rows of plants, but is unable to steer automatically very accurately where the rows are curved. For steering purposes that particular prior art system relies upon information as to which of two plant-contacting sensors (the right-hand sensor or the left-hand sensor), at the front of the same V-guide is deflected by the corn plants.
The reason that the vehicle does not automatically follow curved rows of plants very well is that a relatively large error (i.e., lateral offset of the vehicle from the optimum travel path) has to develop before a sufficient corrective steering command is produced. This cannot be corrected by merely designing the closed feedback loop with higher gain, because the system then becomes unstable. Even when the feedback loop has the maximum gain that is practical in that prior system the vehicle tracks very poorly around turns.
In such a prior art system the accuracy of steering around curves cannot be improved by merely providing an integral feedback term in the steering control system because there would be too much delay both in build-up of the integral bias to produce an adequate steering control signal and in reduction of the integral bias after the crop vehicle comes out of the turn. To improve its steering, the prior system being described employs a bias signal that is controlled manually. When the vehicle enters a turn the operator is required to turn a knob to add enough bias signal to keep the crop vehicle lined up with the rows of crop. Needless to say it is inconvenient for the operator to have to assist the automatic steering system in this way.